From the viewpoint of enhancing thermal efficiency, combined cycle power generation is becoming mainstream in recent thermal power plants. A combined cycle power plant is a power plant comprising a combination of a gas turbine, a steam turbine and a heat recovery steam generator. A high-temperature and high-pressure combustion gas is fed from a combustor to the gas turbine so that the combustion gas, through its expansion, rotates the gas turbine, thereby rotating a power generator. The exhaust gas is then introduced into the heat recovery steam generator, where steam is generated by the thermal energy of the exhaust gas. The steam is fed to the steam turbine which, together with the gas turbine, rotates the power generator.
A heat recovery steam generator is generally constructed to generate steam according to the amount of the heat of an exhaust gas from a gas turbine. These days an increasing number of heat recovery steam generators have an auxiliary combustor for heating an exhaust gas for the following reasons: Due to a lowering of the output of a gas turbine in the summer months, there is a decrease in the amount of an exhaust gas, resulting a decrease in the amount of steam generated in a heat recovery steam generator. The decrease in the amount of steam generated needs to be compensated for. In addition, increasing the amount of steam generated is necessary when steam generated is to be supplied to a site other than a steam turbine, such as a cogeneration plant or a water desalination plant.
The size of an auxiliary combustor, provided in such a recent heat recovery steam generator, is becoming larger these days in order to increase the amount of steam to be supplied. This leads to an increase in the temperature of an exhaust gas in a heat recovery steam generator, which may lower the durability and the reliability of boiler's constituent members. To avoid the problem, a proposal has been made which involves providing auxiliary combustors at a plurality of locations in a heat recovery steam generator (see e.g. Japanese Patent Laid-Open Publication No. 2001-116208).